Instruction manual

AL-800 Instruction Manual
20
power obtained with full microphone gain and a fully clockwise
"ALC SET"
adjustment. For the
cleanest audio, the microphone gain can be reduced until the ALC meter just flickers on voice
peaks.
AM (Amplitude Modulation) Operation
The efficiency of any conventional linear amplifier must remain below half the peak efficiency
under maximum signal conditions (less a safety factor) in AM operation. Since this linear amplifier
has a
peak plate efficiency of nearly 65%, the carrier efficiency must be kept below 30% to maintain
linearity. The power dissipated in the tube anode will be more than twice the carrier output power.
The carrier power must be kept below 150 watts for clean and safe AM operation. With 150 watts
of carrier, the peak envelope power will reach 600 watts with 100% symmetrical modulation. The
modulated AM power output should be limited to 1200 watts peak with non-symmetrical
modulation that enhances the positive peaks.
Audio Distortion
One problem that often occurs when using a linear power amplifier is SSB audio distortion on one
or more bands. This distortion is usually caused by RF feedback from either a poorly designed
antenna or poor RF grounding. REMEMBER THAT A GOOD GROUND FOR DC IS NOT
NECESSARILY GOOD FOR RF. RF circuits require short connections with smooth, wide
conductors. Braiding or woven wire has a very high RF resistance and should be avoided. See the
"Grounding" section on page 14 of this manual. More detailed suggestions on grounding are
available by requesting Ameritron's Tech Bulletin on RF Feedback.
FM (Frequency Modulation), RTTY, and DIGITAL OPERATION
The efficiency of this amplifier will approach 65% in the FM, RTTY and DIGITAL OPERATION
modes. Since these modes do not require linear amplification of complex waveforms, the tuning
can be re-adjusted for maximum power at the desired output power level. This will keep the
efficiency as high as possible.
ALC Metering, Controls, and Adjustments
ALC Metering Functions
The
"MULTIMETER"
switch in the AL-800 has two positions that indicate the functioning of the
ALC circuit. These positions are as follows:
ALC
...
In this position the multimeter measures the output voltage of the ALC detector. The full
scale reading of the ALC detector voltage is 35 volts. It is read directly from the
"ACL / HV"
scale
of the multimeter. The multimeter will indicate the maximum value of ALC voltage available from
the internal ALC circuit. The meter should flick upwards occasionally during normal ALC action in
this position.